@Hobbit76 

The 7590 is now configured with the 192.168.0.1/29 network.

Why /29 why not /24 ?

What's its DHCP server range set to ?

How is the WAN port of the ER605 configured ?

  @MisterW 

The Fritzbox does not require a /24 network. It's just supposed to be a transfer network. She only has one device, the router. A /30 network without DHCP should also be enough, but I also wanted to log in to the Fritzbox, so it has a /29 network with the DHCP range 192.168.0.2 to 192.168.0.6

The WAN connection type is dynamic IP via DHCP.

  @Hobbit76 

The Fritzbox does not require a /24 network. It's just supposed to be a transfer network. She only has one device, the router. A /30 network without DHCP should also be enough, but I also wanted to log in to the Fritzbox, so it has a /29 network with the DHCP range 192.168.0.2 to 192.168.0.6

Ok, that makes sense, my original concern was that the ER605 WAN had been allocated an address that was outside the LAN subnet of the Fritzbox, but that isnt the case.

Okay, so. . . i forgot that the LAN phones are configured in my Fritzbox

So do they have addresses in the 192.168.0.2 to 192.168.0.6 range ?

Given your setup, yes you will have a double NAT which is not ideal for some situations, but you should be able to access anything in the 192.168.0.1/29 subnet from the ER605 LAN. The ER605 will just treat any 192.168.0.1/29 destination address as being outside its lan ( 192.168.178.1/24 ) .and thus it will route it via the WAN port, performing a NAT translation as it does. So now its source is the WAN IP of the ER605 (192.168.0.3) and so is now local to the Fritzbox LAN. Any reply will go back via the reverse route and be un-natted.

So it should just work...

MisterW wrote

The ER605 will just treat any 192.168.0.1/29 destination address as being outside its lan ( 192.168.178.1/24 ) .and thus it will route it via the WAN port, performing a NAT translation as it does. So now its source is the WAN IP of the ER605 (192.168.0.3) and so is now local to the Fritzbox LAN. Any reply will go back via the reverse route and be un-natted.

So it should just work...

  You're right.

My LAN phones are working now. SIP requires a domain and the Omada solution does not provide local DNS. I have configured a Pihole and am using this as the primary DNS. fritz.box is now known as 192.168.0.1 and SIP telephony works.

  @Hobbit76 

SIP requires a domain

That's a new one on me. Is it just a requirement of SIP on the Fritzbox ? 

At least you have it working now